I occasionally prowl discussion boards of people who get injections for butt lift.
There are many accounts of horrible deformities and complications. Please do not do this. The problem is that some people get away with it and they provide a lot of testimonials. Others, however, suffer the consequences and get deformities for life.
See the video below for information on how to get good, reliable results with fat grafting.
I had a post some time ago on Hydrogel, different substance, but same problems.